Why do linguists use square brackets for phones and slash marks for phonemes?

In linguistics, the use of different delimiters for phonetic and phonological transcription is a standard convention used to distinguish between physical speech sounds and abstract linguistic units.1

• Square Brackets [ ]: These are used to enclose phones, which are the actual, observable speech sounds produced by a speaker. Because phones represent the physical reality of speech, they are used in phonetic transcription to capture acoustic details. Depending on the level of detail required, this can range from "broad" transcription (omitting minor details) to "narrow" transcription (using diacritics to record precise qualities like aspiration or length).123

• Slash Marks / /: These are used to enclose phonemes, which are the abstract, functional units of sound that distinguish meaning within a specific language. Phonemes are not physical sounds themselves but rather the mental categories that speakers use to organize language. A single phoneme may be realized by different physical phones (allophones) depending on the context, but the slash marks indicate that the linguist is focusing on the underlying phonological identity rather than the specific phonetic variation.14

This distinction is essential because while phones are continuously variable, phonemes are considered discrete and contrastive units within a language's system.1